The present invention relates to a method for detecting an F-number of an imaging lens and more particularly to a method for detecting effective F-numbers of interchangeable lenses of a single-lens reflex camera.
In a usual lens system of the camera, in case of taking a picture, an amount of exposure is controlled to the best value by adjusting a diaphragm of the camera. It is well known that the effective F-number of the lens varies in accordance with the aperture of the diaphragm. Moreover, when use is made of the single-lens reflex camera which can use various interchangeable lenses, open F-numbers of respective interchangeable lenses are different from each other. In this manner, the F-number is largely varied both when the single lens is used and when the interchangeable lenses are used.
As for a focus detection of such lens systems, an image lateral shift detection method has been known. In this focus detection system, since an amount of the image lateral shift is theoretically varied in accordance with the F-number, it is necessary to detect the F-number and to correct a defocus amount of the lens system by the detected F-number each time the focus detection is performed.
FIG. 1 is a schematic view showing one embodiment of a known method for detecting the F-number of a photographing lens. In FIG. 1, a pin 2 is arranged near a lens mount of an interchangeable lens 1 such that its position denotes an open F-number of the interchangeable lens 1. After coupling the interchangeable lens 1 with a camera body 3, the position of the pin 2 in the camera body 3 is detected and the open F-number of the lens 1 is derived from the thus detected position. However, in this known F-number detection system, since mechanical parts and a space therefor are necessary, the weight of the whole camera system is increased correspondingly and various interchangeable lenses of different makers can not be used if a specification of the pin 2 and its detection device is fixedly determined.
There has been proposed another system for detecting the F-number of the lenses. In this system a switch is arranged in the camera body and every time the lens is exchanged, its F-number is manually set by the switch. However, in this case, it is very cumbersome for a user to handle the switch every time the lens is exchanged, and further, if the user forgets to set the switch corresponding to the F-number of the lens to be used, information about the F-number cannot be obtained precisely.